Abstract: Population Core The projects in the USC-TCORS will require access to population resources with sufficient representation of tobacco product exposures, population subgroups, and longitudinal follow-up. The USC-TCORS Population Core (PC) will provide the Center with a sufficiently large, diverse, and well-followed sample to address the proposed scientific goals. We leverage data from a highly successful 2013-2018 initial funding period, in which we developed and tracked two cohorts with excellent retention. For 2018-2023, PC proposes the following aims. Aim 1. Expand, follow, track, and assess a Center-wide combined cohort. The PC will leverage data collected during the current award from two samples of adolescents, now young adults, for additional follow-up: (i) the Children's Health Study (CHS), which has extensive smoking information collected through schools since early adolescence, and complete information on tobacco product use starting at age 16-18 years; CHS participants will be aged 20-22 in 2018 (N=1,366); and (ii) the Happiness and Health (H&H) study, which has comprehensive tobacco product use information and covariates since enrollment in 2013 at age 14; H&H par- ticipants will be aged 18-19 in 2018 (N=2,534). Using now well-established methods, we will also recruit a new younger sample, the Trends in Tobacco Use Survey (TITUS) study, from 9th grade classrooms (age 14) in schools where CHS and H&H were originally drawn from (N=2,500). We will integrate these three resources into combined Center-wide cohort of 6,400 youth and young adults. The PC will conduct annual surveys on the Center-wide cohort to serve P1 and P3 aims, utilizing a common electronic survey infrastructure developed in partnership with the Administrative Core and ensuring common survey items across projects. The PC will also collect and manage a database of cohort data to be made available for the other projects, Rapid Re- sponse Projects, collaborations with other TCORSs, and junior investigator pilots (from the Career Enhance- ment Core). The database will be used for secondary data analysis projects and to identify populations with selected characteristics for recruitment into P4. The database will also be drawn upon for shared aims regard- ing social media exposure and tobacco use for P1 and P3. Finally, the database will be a resource for addi- tional original data collection projects. Aim 2. Facilitate access to other population resources outside of the Center-wide cohort within the USC Department of Preventive Medicine. The PC will enhance existing connec- tions with and access to population cohorts and archival databases including numerous diverse samples. In sum, PC cohorts will provide an unmatched population resource in terms of diversity, follow-up length, meas- urement intensity, and richness of data. Our resource will complement and provide unique information to other large samples such as the Population Assessment of Tobacco and Health survey by providing population di- versity, product diversity assessment, and tobacco product exposure analysis that is not otherwise available.